Just found this paper, published in 2006:
Applied Radiation and Isotopes
Volume 64, Issue 5, May 2006, Pages 543-550
"Electron inelastic mean free path formula and CSDA-range calculation in biological compounds for low and intermediate energies"
Ayşegül Akar, Hasan Gümüş and Nazmi T. Okumuşoğlu
Low-energy (<20 eV) electrons go surprisingly far in biological tissue: >100 nm in water.